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Investigators have been working at the crash site in California

Virgin Galactic insists safety has always been central to its operations, following the crash of its experimental spacecraft in the US on Friday.

The company said that principle "has guided every decision we have made over the past decade, and any suggestion to the contrary is categorically untrue".

SpaceShipTwo broke up in mid-air during a test flight in California's Mojave Desert, killing one of the two pilots.

Virgin Galactic aims to send tourists on suborbital flights.

US National Transportation Safety Board (NTSB) head Christopher Hart said the cause of the crash was still to be determined, but added that investigators had discovered that a device to slow the craft's descent had prematurely deployed "without being commanded".

He said SpaceShipTwo's fuel tanks and engines showed no signs of being compromised.

NTSB investigators have now found almost all of the parts of the crashed spacecraft as part of an inquiry that could take many months to complete.

"We'll be looking at training issues. We'll be looking at was there pressure to continue testing. We'll be looking at safety culture. We'll be looking at the design, the procedure," Mr Hart told reporters.

"We've got many, many issues to look in to much more extensively before we can determine the cause."

'Time to focus'

In a statement responding to criticism in the media about its approach to safety, Virgin Galactic said "everything we do is to pursue the vision of accessible and democratised space - and to do it safely".

It added: "Just like early air or sea travel, it is hard and complicated, but we believe that a thriving commercial space industry will have far reaching benefits for humanity, technology and research for generations to come.

"Now is not the time for speculation. Now is the time to focus on all those affected by this tragic accident and to work with the experts at the NTSB, to get to the bottom of what happened on that tragic day."

Virgin Galactic founder Sir Richard Branson has said he is "determined to find out what went wrong" and learn from the tragedy.